Even if you're planning a home birth, writing a birth plan for the planned birth and in the event of a transfer will help you consider all your options. So it might be a good thing to do anyway. Realize that if you have to transfer, it is mostly likely going to be an emergent situation, and the hospital staff won't take the time to read your birth plan, so you need to be sure that whomever accompanies you to the hospital knows exactly what you do and don't want and can voice it for you.
I also found it helpful to write a bith plan for all contingencies, even a csection, again because it got me thinking about things I hadn't considered. I knew very little about csections, but as I started researching them to write that contingent birth plan, I learned there was a lot more I needed to know! Like the option of spinal vs epidural, after surgery pain relief, stitches vs staples vs steri strips (some of this might be practitioner preference and might be out of your control, but it's worth researching so if given a choice you are educated about it).
And talk to all your support people about what is truly important to you. For example, in my first birth my birth plan went out the window when the labor became complicated and interventions were necessary. BUT one thing that *was* left in my control was the fact that the baby got no formula, despite being in NICU after birth. It felt good to have one thing that I could cling to. But it took my husband standing up for our decision while I was left behind in the delivery room hemorrhaging and he went with our son to NICU. He said that he got into quite a little match with the neonatologist about it, too! I'm so thankful that he understood that it was really important to me.